Interference of IgM paraproteins in the Olympus AU800 uric acid assay

In the Olympus uric acid procedure, uric acid is converted by uricase to allantoin and hydrogen peroxide, which is reacted in a Trinder reaction to produce a chromophore read bichromatically at 520 and 660 nm. Repeated difficulty was encountered in obtaining uric acid results on samples from myeloma patients with known IgM paraproteins. Large absorbances in sample blanks were due to a visible precipitation observed in the reaction cuvettes.

To alter the Olympus method (OM) to eliminate the interference by IgM, and to verify the modified method (MM).

Dilution of the sample blank by saline was substituted for water in the MM, with small alterations in the reaction timing sequence necessary to accommodate the instrument requirements.

A comparison of uric acid results obtained from nonmyeloma patient samples using the OM and the MM showed a good correlation (r = 0.970), and no statistical difference between the two means using a paired t-test. A similar comparison performed using the samples containing IgA and IgG paraproteins also revealed a good correlation (r = 0.981), and no statistical difference between the two means. Results on IgM containing specimens were assessed indirectly because the samples could not be assayed with the OM. First, removal of detectable levels of proteins using a 20% TCA solution did not affect the measurement of uric acid. Second, protein-free supernatants from IgM containing samples were measured by the OM and compared with the corresponding serum samples measured by the MM. There was good correlation between the two methods (r = 0.945), and no statistical difference between the means using a paired t-test.

The modified method is satisfactory for routine analysis of samples, including those with IgM paraproteins.